Where is the money for welfare coming from if tax from working people is drastically reduced?
The problem is that income tax by workers is by far the largest contribution to the total tax intake of any country. To match the missing tax from replaced workers (including the highest paid) in the US, you'd have to increase tax on corporations by 3-4x, which would kill many businesses. On top of that you'd have far more people taking from the tax pot via welfare.
In the worse case scenario, mass replacement of workers by AI would essentially amount to the destruction of the economy.